1. Field of the Invention
This invention relates to polyurethane coating compositions and more particularly to polyurethane coatings based on polyisocyanatoisocyanurates.
2. Description of the Prior Art
Good polyurethane resin (PUR) materials and coatings can be produced using aromatic polyisocyanates and suitable hydroxyl components. However, they have the disadvantage that they tend to become yellow when exposed to daylight.
Therefore, for demanding applications it is necessary to convert cyclic polyisocyanates, such as isophorone diisocyanate (IPDI) or its oligomers or adducts with compounds of appropriate structure into light-stable PUR systems. End products of this type also have thermal and oxidative stability. However, these resins are not suitable for cold curing and are preferably utilized in hot curing applications. The resins are very compatible, but they do not form extremely solvent resistant coatings.
Aliphatic polyisocyanates, their oligomers and adducts in combination with suitable hydroxyl containing components are well-known reactive systems for cold curing. During hot curing or when stored at high temperature, they tend to yellow. This is also the case with the monomeric diisocyanates as well as with the biuret forms, the isocyanurates and the diol/triol adducts, including the aliphatic diisocyanates known in the literature and the aforementioned derivatives such as, for instance, the hexamethylenrediisocyanate (HDI), the 2,2,4 (2,4,4)trimethylhexamethylenediisocyanate (TMDI) or the 2-methylnonamethylene diisocyanate (MNDI).
Starting from bicyclic and tricyclic isocyanates and using suitable hydroxyl compounds, PUR coatings with a remarkable thermal stability but with a distinct yellowing tendency and limited weatherability are obtained. Tricyclodecane diisocyanate (TCDI) is an example.
Therefore, a need has continued to exist for a polyurethane coating having good heat-stability, a low tendency to become yellow and good stability to weathering.